The invention relates to calendering of paper and to a method, wherein a paper web is passed through a nip formed by a heatable thermo roll and a backing roll.
In calendering, paper is pressed in the nip, whereby the surface of the paper in particular is moulded under the effect of mechanical work and heat. The purpose is to increase especially the smoothness of the paper, and to eliminate variations in thickness. However, in calendering, the paper is also compressed, which decreases the stiffness, the strength, and the opacity.
The plasticity of paper in calendering can be improved by increasing the temperature of the paper. In practice, this is effected so that one of the rolls of the nip is a heatable roll, a so-called thermo roll, which is against the surface of the web that is to be moulded. In the calendering methods used at present, the surface temperature of the thermo roll is in the range of the glass-transition temperature of the paper that is moulded, at the most. The glass-transition temperature is dependent on the paper grade. Moisture decreases the glass-transition temperature, which is why the paper is often moistened before calendering. Typically, the glass-transition temperature is within 150 to 250° C.